Photographic reproduction of halftone screens

ABSTRACT

The method of making, and article of manufacture therefor, of a photographic master wherein an unexposed sheet of orthochromatic film is secured to a rotatable drum, moving the drum longitudinally during rotation, and permitting a narrow light source to be exposed to the film resulting in a sheet of film having a series of extremely close parallel lines, and developing the film. The method of making, and article of manufacture therefor, of a halftone screen wherein an unexposed sheet of orthochromatic film is placed adjacent a photographic master, exposing the master onto the film, removing and replacing the master so that the lines of the master are at 90* to the first position, exposing again the film, and developing the film. The method of making and article of manufacture therefor of a halftone screen wherein an unexposed sheet of orthochromatic plate is placed adjacent a photographic master, exposing the master onto the plate, developing the plate, repeating the procedure to a second plate, placing the two exposed plates adjacent with the lines at 90*.

United States Patent Thompson Edward R. Thompson, North Hollywood,Calif.

Lockheed Aircraft Corporation, Burbank, Calif.

[22 Filed: Nov. 19, 19 9 211 Appl.No.: 878,087

[72] Inventor:

[73] Assignee:

[52] U.S.Cl ..96lll6 [58] Field 61 Search .....96/11 [56] ReferencesCited 7 UNITED STATES PATENTS 2,183,204 12/1939 Reynolds ..96/1162,719,790 10/1955 Monray 96/116 2,468,680 4/1949 Marx ..96/116 2,566,2658/1951 Tuttle 96/116 x 2,757,087 7/1956 Wicklund. 96/116 x 426,2834/1890 Kohl 96/116 x 492,333 2/1893 Levy ..96/116 5] Mar. 7, 1972Primary Examiner-David Klein Attorney-George C. Sullivan and Ralph M.Flygare ABSTRACT The method of making, and article of manufacturetherefor, of a photographic master wherein an unexposed sheet oforthochromatic film is secured to a rotatable drum, moving the drumlongitudinally during rotation, and permitting a narrow light source tobe exposed to the film resulting in a sheet of film having a series ofextremely close parallel lines, and developing the film. The method ofmaking, and article of manufacture therefor, of a halftone screenwherein an unexposed sheet of orthochromatic film is placed adjacent aphotographic master, exposing the master onto the film, removing andreplacing the master so that the lines of the master are at 90 to thefirst position, exposing again the film, and developing the film. Themethod of making and article of manufacture therefor of ahalftone screenwherein an unexposed sheet of orthochromatic plate is placed adjacent aphotographic master, exposing the master onto the plate, developing theplate, repeating the procedure to a second plate, placing the twoexposed plates adjacent with the lines at 90.

2 Claims, 8 Drawing Figures PATENTEUMAR 7 1912 SHEET 1 PF 2 INVIZN'I'OA.EDWARD R. THOMPSON PHOTOGRAPHIC REPRODUCTION OF I-IALFTONE SCREENSBACKGROUND OF THE INVENTION The field of this invention relates tohalftone screens for the reproduction of images by printing or likeduplicating processes. Specifically, the field of this invention relatesto a method of producing a halftone screen and the resulting producedhalftone screen.

In the reproduction of images, such as photographic prints or negativesby printing or like duplicating processes, it has been customary toproduce halftone engravings, that is, engraving plates having a screenstructure composed of a series of regularly spaced dots. In suchhalftone screens, the average size and spacing of the dots in eachincremental portion of the plate vary proportionately with the averageshade value or greyness of the corresponding portion of the image. Thesurfaces of these dots lie in the original plane of the screen andreceive the printing ink from a roller or thelike.

Such halftone screens and their method of manufacture are well known inthe art, so that no detailed description is required here. However, itis also well known that the process of making such photoengravingsinvolves elaborate and expensive equipment and a large number of steps,each of which must be performed with great care and skill in order toobtain a satisfactory halftone screen. Such skill is generally acquiredonly be extended apprenticeship and artisans of the required skill arefrequently unavailable. All of the foregoing factors contribute to thehigh cost of such halftone plates and to the time required for theircompletion.

One of the most widely employed methods of manufacture of a halftonescreen is to draw each of the lines upon a sheet of glass and then toetch by acid the entire sheet. As halftone screens usually have anywherefrom 50 lines per inch to 300 lines per inch, it is readily apparentthat the scribing method would be quite time consuming. Actually, tomake an average size screen it would take a skilled artisanapproximately 1 full week in time to make a halftone screen using thescribing method. Another disadvantage of the scribing method is that ifa mistake is made upon the glass sheet, frequently the entire sheet hasto be discarded. If the mistake can be corrected, such is very difficultas well as extremely time consuming.

It would be of particular advantage to be able to produce halftonescreens avoiding the conventional costly procedure of scribing the linesdirectly upon a sheet of glass.

SUMMARY OF THE INVENTION This invention pertains to the method of makingand articles of manufacture of halftone screens through the process ofphotography. It is first necessary to make a photographic master fromwhich the halftone screens are formed. The master is formed by placingan unexposed sheet of film upon a drum and rotating the drum at aconstant predetermined velocity. A light source which produces a beam oflight the thickness of the line to be produced is exposed upon the film.Then, expose the light source to the film and continuously move thelight source longitudinally along the length of the film Remove the filmfrom the drum, develop the film, thereby producing a section of filmwith a plurality of closely spaced parallel lines. Shape the section offilm into a square with the sides of the square being at a 45 angle withthe lines.

To produce a halftone screen from an orthochromatic glass plate, anunexposed plate is placed in contact with the master. Expose the masteronto the plate by means of a light source, thereby producing a platewith a plurality of closely spaced parallel lines when developed. Repeatthe procedure with another plate. Place the plates together forming aright-angled grid pattern.

To produce a halftone screen from a section of orthochromatic film, anunexposed section of film is placed in contact with the master. Exposethe master onto the film by means of a light source. Remove the masterand replace it at an angular displacement of 90 to the first positiori.Expose the film again by means of the light source. Remove the master,develop the film thereby producing a single section of film with asquare grid pattern.

One of the objects of this invention is to provide a method of making ahalftone screen which is much faster than heretoforepossible while stillmaintaining the quality of the screen.

Another object of this invention is to provide an inexpensive method ofproducing halftone screens and plates.

Another object of this invention is to provide a halftone screen whichfacilitates the correction of mistakes as opposed to the conventionalscribing method obtained screens.

I BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of therotatable drum and light source apparatus to be employed to produce aphotographic master which is to be used to make halftone screens andplates;

FIG. 2 is aplan view of the section of film to produce a photographicmaster showing the manner in which the master is formed therefrom;

FIG. 3 is an exploded perspective view of the plate composite type ofhalftone screen;

FIG. 4 is a magnified perspective view of a portion of the composite ofFIG. 3;

FIG. 5 is an enlarged portion in plan view of the film type of halftonescreen;

FIG. 6 is a cross-sectional view through the orthochromatic p'latecomposite type of halftone screen;

FIG. 7 is a cross-sectional view through the orthochromatic film type ofhalftone screen being retained between a pair of clear glass plates; and

FIG. 8 is a cross-sectional view through the contacting relationship ofthe master and film to expose the master upon the film.

DETAILED DESCRIPTION OF THE SHOWN EMBODIMENTS Referring particularly tothe drawings, there is shown in FIG. I one form of apparatus 10 whichcan be employed to produce a photographic master of the type employed inthe invention. It is to be understood that a photographic master is anexposed rectangular sheet of transparent film, with a plurality ofparallel black lines ruled thereon at a 45 angle to the longestdimension. The lines are equal in width with the result being 50 percentblack (opaque) lines and 50 percent open (transparent) space. Masterscan range from 25 lines per inch to 300 lines per inch. A photographicmaster of lines per inch is used in the method of the invention toproduce an 85- line halftone screen. It is to be further understood thatthe apparatus 10 is only one of several different readily availabledevices which could be employed to make photographic masters of the typeherein considered. The apparatus 10 itself forms no part of the claimedinvention.

Upon drum 12 a strip of unexposed photographic film 14, preferably ofthe orthochromatic type, is wound with the ends thereof held together byadhesive tape 16. It should be understood that the process is carriedout in darkness or other ambient conditions appropriate to the handlingof unexposed photographic films. Drum 12 is rotatable by shaft 18 withinfore bearing 20 and aft bearing 22. Fore bearing 20 is retained within afore stanchion 24 supported upon surface 26. Aft bearing 22 is retainedwith gear housing 28 which is supported upon an aft stanchion 30 whichis also supported upon surface 26. An optical head 32 containing a lightsource 34 is supported upon an optical head housing 36. Housing 36 isslidably supported upon guide rods 38 and 40 which are held withinstanchions 24 and 30. Optical head 32 may be translated along rods 38and 40 by a leadscrew 42 which is rotatably supported within stanchions24 and 30. Leadscrew 42 is rotated by an appropriate gear mechanism (notshown) within gear housing 28. The rotation of screw 42 and drum 12 aresynchronized through the gear mechanism to provide a predeterminednumber of lines per inch on the master 14. A single motor 44 providesrotation of both the drum and leadscrew 42. The lens 46 is masked toform a narrow beam of light having a width corresponding to the desiredline width on the master 14.

To produce a photographic master, with a strip of unexposed film 14 upondrum l2 and the optical head 32 located at the far left edge of the film14, the light source is activated. By film is meant a thin, flexible,transparent, optically uniform sheet upon which is carried a coatingcomprising a light-sensitive emulsion. Motor 44 is activated causingdrum l2 and leadscrew 12 to rotate. The beam of light through the maskedlens 46 exposes a line on the film. Rotation of drum l2 and screw 42 iscontinuous until the entire strip of film 14 is exposed. With the film14 fully exposed, it is removed from drum 12 and developed byconventional developing techniques. It has been found that to facilitateease of use of the master 14, the master is formed in the shape of asquare 48 (or rectangle) with the edges thereof at 45 to the ruled linesupon the film 14.

The photographic master may now be used to produce a halftone screeninaccordance with the method of the invention. To produce a halftonescrew from a pair of glass orthochromatic plates, one such plate 50 isplacedv in a vacuum frame, with its photosensitive emulsion 54 side up.The plate 50 is of the standard and well-known type used in professionalphotography. The photographic master 14 produced in the manner describedin the preceding paragraph is placed upon the plate 50, emulsion sidedown. Remove the air from the vacuum frame. The vacuum frame may be ofany suitable and well-known construction, such as is conventionally usedin making photolitho plates and the like. Expose the master 14 and plate50 to a light source. The type of light source or time of exposure is amatter of choice, it only being required that complete exposure occur,not insufficient or excess exposure. lnsufficient exposure will causethe lines to be thin and light in color. Excess exposure will cause thelines to become blurred and not definite at the edges. Open the vacuumframe to the air and remove the photographic master and the exposedorthochromatic plate 50. Develop the plate by conventional methods anddry. A second glass orthochromatic plate 52 is then prepared in themanner just described. The pair of plates thus formed, and havingidentical line ruling, are then combined into an integral structure. Toaccomplish this, place the first plate 50 in a wooden rack with theemulsion 54 side up. Cover the plate 50 with a transparent adhesive suchas Canada Fir Balsam (Balsam has substantially the same index ofrefraction as glass). Place the second plate 52 on top of the Balsamwith emulsion 56 side down. Press out excess Balsam and let dry untilthe plates 50 and 52 are cemented together. Remove the plates 50 and 52from the rack and place a metal binding around all four edges of theresulting halftone screen. The screen is now ready for use.

A second embodiment of the invention utilizes a photosensitive emulsionsupported on a plastic sheet (viz, conventional orthochromatic film) inlieu of sensitized glass plates. To form a halftone screen fromorthochromatic film, place an unexposed sheet of orthochromatic film 58in a vacuum frame, emulsion 60.side up. Place the photographic master 62emulsion 64 side up. Place the photographic master 62 emulsion 64 sidedown upon the film (emulsion to emulsion). Activate the vacuum frame.Contact print by exposing the orthochromatic film from a light sourcethrough the photographic master 62. Return air to the vacuum frame. Pickup the master 62 and turn it over. The film 58 and the master 62 willnot now be emulsionto-emulsion. Because the ruled lines of the master 62are ruled at a 45 angle with respect to the edges of the master 62 byturning the master 62 over the lines of the master 62 are now 90 to theexposed lines upon the film 58. Again remove the air from the vacuumframe and reexpose the master 62 upon the film 58, keeping the exposuretimes exactly the same. Return air to the frame, remove and store themaster 62, and develop the film 58. After the film 58 is dried at roomtemperature, place the film 58 upon a sheet of polished plate glass 6covered with Canada fir Balsam, or other suitable optical cement,located within a wooden rack. Place the cement on the free side of film58 and place another piece of polished plate glass 68 upon the film 58.Remove excess cement and permit to dry. Remove the film 58 and glass 66and 68 from the rack and place a metal binding around the four edges ofthe resulting halftone screen. The photographic halftone screen is nowready for use.

While the foregoing examples refer to orthochromatic" film (this beingthe preferred type) it should be understood that other types ofsensitized emulsions may be employed, as will be obvious to those versedin the art. High contrast, fine grain emulsions are preferred.

The screens produced by the method of this invention are superior tothose produced by direct contact-printing copying of halftone screenmasters, by reason of the fact that unlike prior screens the square dotsdo not have rounded corners which would degrade the resolution.

What is claimed is: l. The method of making a halftone screen comprisingthe steps of: I

placing a strip of unexposed photographic film about the periphery of arotatable drum; rotating said drum and said film about the axis of saiddrum; directing a narrow, fixed intensity, beam of light onto therotating film; longitudinally moving said light beam, with respect tosaid axis, in synchronism with the rotation of said drum so as to exposeside-by-side lines on said strip of film which are spaced apart by adistance equal to the width of said line; removing the exposed film-.strip from said drum and developing said film strip so as to provide aphotographic master having alternating opaque and transparent parallellines; Y placing said photographic master upon a first unexposedorthochromatic glass photographic plate; contact printing the linepattern of the photographic master onto said first plate; removing saidphotographic master from said first plate; placing said photographicmaster upon a second unexposed orthochromatic glass photographic plate;contact printing the line pattern of said photographic master onto saidsecond plate; removing said photographic master from said second plate;developing the first and second plates so as to provide a pair of plateshaving alternating opaque and translucent parallel lines thereon; andcementing emulsion surfaces of said first and' second developed platestogether, with a transparent adhesive having substantially the sameindex of refraction as said glass, in confronting contact so that theline patterns of each are substantially orthogonal with respect to oneanother. 2. The method of making a halftone screen comprising' the stepsof:

placing a strip of, unexposed photographic film about the periphery of arotatable drum; rotating said drum and said film about the axis of saiddrum; directing a narrow, fixed intensity, beam of light onto therotating film; longitudinally moving said light beam, with respect tosaid axis, in synchronism with the rotation of said drum so as to exposeside-by-side lines on said strip of film which are spaced apart by adistance equal to the width of said line; removing the exposed filmstrip from said drum and developing said film strip so as to provide aphotographic master having alternating opaque and transparent parallellines; placing said photographic master upon a sheet of unexposedphotographic film in a first orientation; contact printing the linepattern of said photographic master onto the unexposed film sheet; I I

6 and,

developing the photographic film sheet so as to provide a screen oforthogonally aligned opaque lines with transparent intertices.

2. The method of making a halftone screen comprising the steps of:placing a strip of unexposed photographic film about the periphery of arotatable drum; rotating said drum and said film about the axis of saiddrum; directing a narrow, fixed intensity, beam of light onto therotating film; longitudinally moving said light beam, with respect tosaid axis, in synchronism with the rotation of said drum so as to exposeside-by-side lines on said strip of film which are spaced apart by adistance equal to the width of said line; removing the exposed filmstrip from said drum and developing said film strip so as to provide aphotographic master having alternating opaque and transparent parallellines; placing said photographic master upon a sheet of unexposedphotographic film in a first orientation; contact printing the linepattern of said photographic master onto the unexposed film sheet;relocating said photographic master upon said film sheet so that theline pattern of said photographic master is orthogonal with respect tothe first orientation; thereafter contact printing said photographicmaster in its relocated position onto said film sheet; removing saidphotographic master from said film sheet; and, developing thephotographic film sheet so as to provide a screen of orthogonallyaligned opaque lines with transparent intertices.